The present invention relates generally to gas discharge devices, and more particularly to the cathode construction found in such devices.
A laser typically employs an unheated or cold cathode which is secured to a laser body or housing as a source of electron emission for laser operation. The body of the laser may be composed of glass or glass like materials, generally having low coefficients of thermal expansion. The cathode may be composed of a metal or metal-alloy material well known in the art, for example aluminum. The cathode is generally secured to the laser body by a gas tight seal, and is adapted to be connected to a negative electrical potential source.
In gas lasers having a limited gas supply, cathode sputtering is one of the major causes of shortened laser life. In a heliumneon gas laser, positively charged gas ions of the plasma are attracted to the negatively charged cathode, and release negatively charged electrons. Unfortunately, the positively charge ions can dislodge cathode material molecules from the active electron emitting surface of the cathode. This phenomenon is usually referred to as cathode sputtering. For gas laser applications, cathode sputtering results in decreased laser life. As a result of cathode sputtering, the dislodged cathode material can, in turn, trap gas molecules to the walls of the cathode and the walls of the laser cavity. If the supply of gas needed for lasing is limited, the reduction of available gas ions can cause the laser action to cease.
Metallic cathodes, particularly aluminum cathodes, have been widely used in the art for gas lasers. An aluminum cathode generally has a cathode emitting surface coated with a thin layer of oxide to prevent cathode sputtering. During the cathode manufacturing process, a layer of oxide is formed naturally by exposing a cleaned aluminum cathode emitting surface to an oxygen plasma with the aluminum cathode connected as the cathode in an electrical circuit. A thin layer of oxide is formed on the aluminum electron emitting surface due to the pressure of oxygen and oxygen ions hitting the cathode surface.
Aluminum cathodes having the oxide layer have improved laser life above that of uncoated aluminum due to increased resistance to sputtering. This is so since the oxide layer is generally harder than the aluminum. Nevertheless, irregularities in the emitting surface of the cathode can result in localized ion flow which in time breaks down the oxide layer, and begins localized sputtering of the cathode resulting in extinction of the laser.
U.S. Pat. No. 4,672,623 to Mellum, et al. entitled "CATHODE CONSTRUCTION FOR A LASER" discloses a cathode for a ring laser comprising a housing composed of substantially a nickel-iron composition. The cathode in Mellum, et al. includes an inner cavity with a coating of low sputter electrically conductive material thereon.
Another example of a prior art cathode is disclosed in U.S. Pat. No. 4,853,940 to Ford, et al. which is entitled "LASER CATHODE COMPOSED OF IMBEDDED LASER GAS MOLECULES". U.S. Pat. No. 4,853,940 discloses a long life cathode for laser generators consisting of a cathode body having an electron emitting surface in which lasing gas molecules are embedded.
Yet another example of a laser cathode is disclosed in U.S. Pat. No. 4,910,748 to Ford, et al. which is entitled "LASER CATHODE COMPOSED OF OXIDIZED METALLIC PARTICLES". U.S. Pat. No. 4,910,748 discloses a cathode for laser generators consisting of a monolithic body of an agglomeration of oxidized metallic particles of beryllium or aluminum. The Ford, et al. '748 patent uses a beryllium powder available from Brush Wellman, Inc. which includes 98% beryllium, a maximum of 2% beryllium oxide and trace amounts of other elements.
As reported in an article entitled The Secondary Electron Emission Characteristics of Oxidized Beryllium Cathodes by Ritz, et al., US Naval Research Laboratory, Surface and Interface Analysis, Volume 11, 389-397 (1988), carbon has been found to significantly lower the secondary electron emission of beryllium. However, carbon can be reduced by several hundred atomic layers at the surface of the cathode by sputtering of the cathode by oxygen, argon, helium and/or neon. Ritz, et al. teaches a method for reducing a carbon layer on such cathodes, but does not recognize the importance of increasing the BEO content in the cathode as is recognized by the instant invention. The Ritz, et al. article is incorporated herein by reference.